Light source device, projector, and driving method of discharge lamp

ABSTRACT

To prevent biased consumption of electrodes in a discharge lamp and to prevent biased precipitation of the electrode material, a light source is provided. The light source device has a discharge lamp that emits light by discharge between a first electrode and a second electrode; and a driver that supplies alternating current to the first and the second electrodes so as to maintain the discharge, and changes duty ratio of the alternating current in accordance with predetermined pattern. The predetermined pattern includes a plurality of section periods for which the duty ratio maintains mutually different values for a predetermined period.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the priority based on Japanese PatentApplication No. 2007-322927 filed on Dec. 14, 2007, Japanese PatentApplication No. 2008-204658 filed on Aug. 8, 2008, Japanese PatentApplication No. 2007-325592 filed on Dec. 18, 2007, and Japanese PatentApplication No. 2008-204681 filed on Aug. 7, 2008 the disclosures ofwhich are hereby incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light source device equipped with adischarge lamp that has a pair of electrodes, the driving method of sucha discharge lamp, and a projector that incorporates such a dischargelamp.

2. Description of the Related Art

As a lighting method for a high intensity discharge lamp, U.S. Pat. No.6,815,907 discloses to supply alternating lump current to a highintensity discharge lamp. In the disclosure, absolute value of thealternating lump current is roughly fixed, and the pulse widthmodulation of the alternating lamp current is performed. Specifically,it is disclosed to set lighting frequency to 40 Hz to 5 kHz, and tomodulate the pulse width ratio of the positive pulse and negative pulsewith a sine wave of 0.1 Hz to 100 Hz (specifically, 50 Hz) which islower than the lighting frequency.

However, even when modulating the pulse width of an alternating lampcurrent as disclosed in U.S. Pat. No. 6,815,907, when the pulse widthmodulation frequency is high, a steady convection flow is formed at theinside of the high intensity discharge lamp, and biased consumption ofthe electrode and biased precipitation of the electrode materials mayoccur.

SUMMARY

An object of the present invention is to prevent biased consumption ofthe electrodes and to prevent biased precipitation of the electrodematerial by suppressing the formation of a steady convection flow at theinside of the lamp in an apparatus such as a light source device and aprojector equipped with the light source device.

According to an aspect of the present invention, a light source deviceis provided. The light source device has: a discharge lamp that emitslight by discharge between a first electrode and a second electrode; anda driver that supplies alternating current to the first and the secondelectrodes so as to maintain the discharge, and changes duty ratio ofthe alternating current in accordance with predetermined pattern, thepredetermined pattern including a plurality of section periods for whichthe duty ratio maintains mutually different values for a predeterminedperiod.

With the aforementioned light source device, the driver changes dutyratio of the alternating current in accordance with the predeterminedpattern including a plurality of section periods. In each sectionperiod, the duty ratio maintains mutually different values for apredetermined period. Accordingly, it is possible to temporarily fix theduty ratio with the plurality of section periods included in thepredetermined pattern, while changing the duty ratio. In other words, itis possible to fluctuate the heat state of both electrodes and theirperiphery with a relatively long time scale, so it is possible to avoidformation of a steady convection flow in the discharge lamp. As aconsequence, it is possible to suppress biased consumption of bothelectrodes and biased precipitation of the electrode materials.Additionally, in this case, it is possible to enlarge fluctuation of theheat state of both electrodes and their periphery as a difference in thestates for each section period, so it is possible to efficientlysuppress biased consumption of both electrodes and biased precipitationof the electrode materials.

The present invention may be reduced to practice in various modes, forexample, an driving apparatus or method of a discharge lamp; a lightsource apparatus using a discharge lamp or a control method thereof; animage display apparatus such as a projector using such a method orapparatus; a computer program for carrying out the functions of such amethod and apparatus; a recording medium having such a computer programrecorded thereon; a data signal containing such a computer program andembodied in a carrier wave, and so on.

These and other objects, features, aspects, and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiments with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional diagram showing a conceptual description ofthe configuration of the light source device;

FIG. 2 is a block diagram schematically showing the configuration of thelight source driver;

FIG. 3 is an enlarged cross sectional diagram for describing theconvection flow in the discharge space;

FIG. 4 is a graph describing the pattern for modulating the duty ratioof the alternating current;

FIG. 5A and 5B are graphs for describing the waveform of the alternatingcurrent actually supplied;

FIG. 6A and 6B are graphs for describing an exemplary variation of themodulation of the duty ratio;

FIG. 7 is a flow chart for describing the operation of the light sourcedriver;

FIG. 8 is a schematic drawing for describing the configuration of aprojector incorporating the light source device;

FIG. 9 is a graph for describing the modulation of the duty ratio of thealternating current in the second embodiment;

FIG. 10A shows a modulation pattern of alternating current when the dutyratio changing amount is set to 5%;

FIG. 10B shows changing of the shape of the electrodes when the dutyratio changing amount is set to 5%;

FIG. 11A shows a modulation pattern of alternating current when the dutyratio changing amount is set to 10%;

FIG. 11B shows changing of the shape of the electrodes when the dutyratio changing amount is set to 10%;

FIG. 12A shows a modulation pattern of alternating current when the dutyratio changing amount is set to 20%;

FIG. 12B shows changing of the shape of the electrodes when the dutyratio changing amount is set to 20%;

FIG. 13 is a graph for describing the modulation of the duty ratio ofthe alternating current in the third embodiment;

FIG. 14 is an explanatory drawing showing a first variation of themodulation pattern; and

FIG. 15 is an explanatory drawing showing a second variation of themodulation pattern.

DESCRIPTION OF THE PREFERRED EMBODIMENT First Embodiment

Referring to the drawings, apparatuses as a first embodiment of thepresent invention such as the light source device are described.

FIG. 1 is a cross sectional diagram showing a conceptual description ofthe configuration of the light source device 100. With the light sourcedevice 100, the light source unit 10 has a discharge light emission typedischarge lamp 1 as a discharge lamp, a reflector 2 which is an ellipticmain reflective mirror, and a sub-mirror 3 which is a sphericalreflective sub-mirror. As described in detail later, the light sourcedriver 70 is configured as an electrical circuit for controlling lightemission in the desired state by supplying alternating current to thelight source unit 10.

In the light source unit 10, the discharge lamp 1 has a tube body 11constituted by a translucent silica glass tube with the sealed centerpart bulged in a sphere shape. The light for illumination is emittedfrom the tube body 11. The discharge lamp 1 also has a first sealingportion 13 and a second sealing portion 14 that extend along the axialline passing through both ends of this tube body 11.

In a discharge space 12 formed inside the tube body 11, the tip of afirst electrode 15 and the tip of second electrode 16 are arrangedseparated by a specified distance. Both of the electrodes 15 and 16 aremade of tungsten. As discharge medium, gas containing a rare gas, ametal halogen compound and so on is enclosed in the discharge space 12.The sealing portions 13 and 14 that extend to both ends of this tubebody 11 have molybdenum metal foils 17 a and 17 b which are electricallyconnected to the base of the first and second electrodes 15 and 16provided at the tube body 11. Airtight seal of the discharge space tothe outside is achieved by both of sealing portions 13 and 14. When thelight source driver 70 supplies an alternating pulse current to leadlines 18 a and 18 b which are connected to the metal foils 17 a and 17b, an arc discharge occurs between the pair of electrodes 15 and 16.This makes the tube body 11 to emit light at high brightness.

The sub-mirror 3 covers a part of the tube body 11 of the discharge lamp1. Specifically, the sub-mirror 3 covers approximately half of the lightflux emitting side (front side) where the second electrode 16 islocated. The sub-mirror 3 is closely-arranged to the tube body 11. Thesub-mirror 3 is an integrated molded product made of silica glass. Thesub-mirror 3 has a reflective portion 3 a, and a support 3 b whichsupports the reflective portion 3 a. The reflective portion 3 a returnslight flux emitted to the front side from the tube body 11 of thedischarge lamp 1 to the tube body 11. The support 3 b is fixed to theperiphery of the second sealing portion 14. The second sealing portion14 is inserted through the support 3 b, and the support 3 b holds thereflective portion 3 a in a state aligned in relation to the tube body11.

The reflector 2 covers another part of the tube body 11 of the dischargelamp 1. Specifically, the reflector 2 covers approximately half of theopposite side of light flux emitting side (back side) where the firstelectrode 15 is located. The reflector 2 is arranged facing opposite tothe sub-mirror 3. This reflector 2 is an integrated molded product madeof crystallized glass or silica glass. The reflector 2 has a neck 2 athrough which the first sealing unit 13 of the discharge lamp 1 isinserted, and an elliptical curved surface reflective portion 2 b whichwidens from the neck 2 a. The first sealing portion 13 is insertedthrough the neck 2 a. The neck 2 a hold the reflective portion 2 b in astate aligned in relation to the tube body 11.

The discharge lamp 1 is arranged along the rotationally symmetric axisor the optical axis of the main reflective portion 2 b which correspondsto the system optical axis OA. The light emitting center O between thefirst and second electrodes 15 and 16 within the tube body 11 isarranged so as to roughly match the first focal point F1 of theelliptical curved surface of the reflective portion 2 b. When thedischarge lamp 1 is lit, the light flux emitted from the arc at theperiphery of the light emitting center O of the tube body 11 isreflected by the reflective portion 2 b, or is reflected further by thereflective portion 2 b after being reflected by the reflective portion 3a. The light flux converged at the second focal point F2 of theelliptical curved surface. In other words, the reflector 2 and thesub-mirror 3 have a roughly axially symmetrical reflective curvedsurface in relation to the system optical axis OA, and the pair ofelectrodes 15 and 16 are arranged so that axial cores of the electrodeshafts roughly match the system optical axis OA.

The discharge lamp 1 is produced by supporting the first and secondelectrodes 15 and 16 which are fixed at the ends of the metal foils 17 aand 17 b in a silica glass tube, softening and shrinking the silicaglass tube at the parts corresponding to both sealing portions 13 and 14from the periphery by heating with a burner (shrink sealing). Thedischarge lamp 1 is fixed to the reflector 2 by inserting the firstsealing portion 13 into the neck 2 a of the reflector, filling aninorganic adhesive C by injection, and solidifying the adhesive C. Thedischarge lamp 1 is also fixed to the sub-mirror 3 by inserting thesecond sealing portion 14 of the discharge lamp 1 into the support 3 bof the sub-mirror 3, filling an inorganic adhesive C by injection, andsolidifying the adhesive C.

FIG. 2 is a block diagram schematically showing the configuration of thelight source driver 70 for lighting the light source unit 10 shown inFIG. 1 in a desired state.

The light source driver 70 generates alternating current for causingdischarge between the pair of electrodes 15 and 16 shown in FIG. 1, andcontrols the state of supplying alternating current to both electrodes15 and 16. The light source driver 70 has a lighting unit 70 a, acontrol unit 70 b, and a DC/DC converter 70 c. Here, as an example, acase is described of the light source driver 70 using an external powersupply. In this case, the light source driver 70 is connected to anAC/DC converter 81, and the AC/DC converter 81 is connected to acommercial power supply 90. The AC/DC converter 81 converts thealternating current supplied from the commercial power supply 90 todirect current.

The lighting unit 70 a is a circuit for driving and lighting the lightsource unit 10 of FIG. 1. With the lighting unit 70 a, the drivewaveform output from the light source driver 70 is adjusted. Here, thedrive waveform is defined by the output current or voltage frequency,amplitude, duty ratio, positive/negative amplitude ratio, waveformcharacteristics and so on. To the light source unit 10, the lightingunit 70 a may output drive current with various waveforms such as asquare wave, a superimposed wave for which a triangular wave issuperimposed on the square wave.

The control unit 70 b is a circuit constituted from, for example, amicrocomputer, memory, sensor, interface, and so on. The control unit 70b is driven by appropriate drive voltage generated by the DC/DCconverter 70 c as the power supply. The control unit 70 b has aoperation control unit 74 for controlling the operating state of thelighting unit 70 a, a determination unit 75 for determining the state ofthe discharge lamp 1, and a data storage unit 76 for storing variousinformation on the power feed state of the lighting unit 70 a such asthe power feed conditions.

The operation control unit 74 operates according to the program storedin the data storage unit 76 and so on. The operation control unit 74selects an power feed condition suited for the current state of thedischarge lamp 1 from the initial operating power feed conditions andthe steady operating power feed conditions stored in the data storageunit 76. The operation control unit 74 performs the initial operation orthe steady operation according to the selected power feed condition onthe lighting unit 70 a. Note that the operation control unit 74 workstogether with the lighting unit 70 a, feeds power to the discharge lamp1, and functions as a current drive device for performing the necessarylighting operation. With this embodiment, the steady operation means anoperation to supply steady energy to the first electrode 15 and thesecond electrode 16. The initial operation means an operation duringstartup period before performing the steady operation to supply energyto the first electrode 15 and the second electrode 16 in an operationdiffers from the steady operation.

The determination unit 75 determines degradation stage the dischargelamp 1 based on the state of the discharge lamp 1, specifically thecumulative lighting time of the discharge lamp 1, the voltage applied tothe discharge lamp 1, and so on.

In addition to the program for operating the operation control unit 74,the data storage unit 76 stores a plurality of initial power feedconditions as a mode of the initial operation of the discharge lamp 1,and stores a plurality of steady power feed conditions as a mode of thesteady operation of the discharge lamp 1. In specific terms, the datastorage unit 76 stores various parameters including the setting valuesof the current value and frequency during startup or warming upperformed as the initial operation. Also, the data storage unit 76stores various parameters during steady operation which relate to thecurrent value, frequency, duty ratio, modulation pattern of the dutyratio, triangular wave rising rate and so on. The duty ratio modulationpattern includes parameters such as the modulation frequency of the dutyratio, and the length of section period, and the variation range of theduty ratio. Here, the triangular wave rising rate is a type of parameterthat defines the waveform characteristics. The triangular wave risingrate represents the ratio of the maximum current value to the averagecurrent value during the half cycle of the superimposed wave for which atriangular wave is superimposed on a square wave.

FIG. 3 is an enlarged cross sectional diagram for describing theconvection flow in the discharge space 12 formed in the tube body 11 ofthe discharge lamp 1 of FIG. 1. At the inside of the tube body 11, bothof the first and second electrodes 15 and 16 have tip 15 a and 16 a,large-diameter portions 15 b and 16 b, and shafts 15 c and 16 c. Duringthe steady operation in which the rated operation of the discharge lamp1 is performed, an arc AR is formed by the arc discharge between thetips 15 a and 16 a of the pair of electrodes 15 and 16. This arc AR andits nearby area become very high in temperature. As a result, inside ofthe discharge space 12, a convection flow AF that flows upward from thearc AR is formed. This convection flow AF moves along the upper halfpart 11 b when it reaches the top part 11 a of the tube body 11, and iscooled while flowing down around the shafts 15 c and 16 c of electrodes15 and 16. The convection flow AF which flows down in this way furtherflows down along the bottom half part 11 c of the tube body 11. Bymutually colliding the convection flow AF beneath the arc AR, theconvection flow AF rises and returns to the arc AR at upside. In otherwords, the convection flow AF is formed and circulates in the peripheryof both electrodes 15 and 16. Since such a convection flow AF containselectrode material which is melted and evaporated by the arc AR, thesteady convection flow causes deposition or precipitation of theelectrode material on the shafts 15 c and 16 c, and growth of whisker ina needle shape. In consequence, unintentional discharge toward the upperhalf part 11 b may occur. Such unintentional discharge causesdegradation of the inner wall of the tube body 11 and decrease of theproduct life of the discharge lamp 1. Also, when lighting with a singledrive waveform continues for a long time, a temperature distribution ofthe electrode is fixed for a long time. As a result, the asymmetry ofthe electrodes that occurs as a change of state over time tends to befomented together with time. So as to resolve such a problem, the dutyratio of the alternating current supplied between the first and secondelectrodes 15 and 16 is changed, and by cyclically fluctuating thetemperature distribution of the electrodes 15 and 16, biased degradationof the electrodes is prevented, and furthermore, with a temperaturedifference of several hundred K that occurs with the left and rightelectrodes 15 and 16, formation of a steady convection flow AF insidethe discharge space 12 is prevented.

In specific terms, with a cycle sufficiently long compared to the cycleof the current waveform of the fixed frequency (lighting frequency)supplied to the pair of electrodes 15 and 16, the duty ratio of thecurrent waveform is cyclically changed. At this time, in order toenlarge fluctuation of the convection flow AF, with the pattern forchanging the duty ratio of the current waveform supplied to bothelectrodes 15 and 16, the duty ratio for the plurality of sectionperiods, which are included in the cyclical period (the modulationcycle), are maintained to two or more different values during a periodgreater than a predetermined length. In other words, the duty ratio ofthe current waveform supplied to the electrodes 15 and 16 is graduallychanged to discrete values, and is increased and decreased cyclically.

To describe specific operation conditions, it is assumed that thelighting frequency supplied to both electrodes 15 and 16 is set toapproximately 60 Hz to 500 Hz for example. It is also assumed that eachsection period which is included in the duty ratio modulation cycle isset to approximately 1 second or greater, and the duty ratio in eachsection period is maintained at a fixed value. For example, bysegmenting the duty ratio into 10 and setting each section period to 1second, total length of each period, or the modulation cycle of the dutyratio becomes 10 seconds. By modulating the duty ratio with this kind ofmodulation pattern, it is possible to gradually fluctuate the heat stateof both electrodes 15 and 16 and their vicinity with a long span so asto affects the convection flow AF. Thus, formation of a steadyconvection flow AF inside the tube body 11 of the discharge lamp 1 maybe avoided. As a result, it is possible to prevent growth of electrodematerial whisker at unintended locations of both electrodes 15 and 16,and furthermore, to prevent rapid progression of shape degradation ofboth electrodes 15 and 16. To avoid formation of the steady convectionflow AF, it is preferable that the length of the section period is lessthan or equal to 1 minute.

In order to estimate minimal length of the section period, we performedexperiments to estimate transient characteristics of electrodetemperature when the duty ratio was changed. In the experiments, thesection period was set to 5 second, the duty ratio was changed from 20%to 80% in 10%, and the temperature of the electrode 15 was measured.When we increase the duty ratio from 40% to 50%, the temperature of theelectrode 15 rose about 40 K. After the lapse of about 0.5 second fromthe change of the duty ratio from 40% to 50%, the temperature of theelectrode 15 became stable. Before the stable period, a quasi-stableperiod in which the temperature rises gradually was observed. The risingamount of temperature At during the transient period where thetemperature changes rapidly may be estimated as the temperaturedifference between the temperature during the stable period before thechange of the duty ratio and the temperature at the beginning of thequasi-stable period after the change of the duty ratio. It is possibleto change the electrode temperature sufficiently, if the time in whichthe temperature changes in one-half of the temperature difference Atelapses. In the experiments, after the lapse of about 0.1 second fromthe change of the duty ratio, the temperature changed in one-half of thetemperature difference At. With the experiments, it may be concludedthat preferable length of the section period is longer than or equal to0.1 second, and much preferable length of the section period is longerthan or equal to 0.5 second.

FIG. 4 is a graph describing the pattern for modulating the duty ratioof the alternating current supplied to the pair of electrodes 15 and 16.The horizontal axis represents time, and the vertical axis representsthe duty ratio. As is clear from the drawing, the duty ratio of thealternating current supplied to both electrodes 15 and 16 increases anddecreases cyclically with the cycle Tm by changing the duty ratio at afixed amount for each change of section period. Each modulation cycle Tmconsists of the anterior half period H1 for which the anode period ofthe first electrode 15 is relatively long and the posterior half periodH2 for which the anode period of the second electrode 16 is relativelylong. In the pattern for modulating the alternating current duty ratioshown in FIG. 4, one cycle has 17 section periods of section periods P1to P17. The duty ratio where the anode period of the first electrode 15(hereafter called “anode duty ratio”) is changed in 9 stages of 5% stepswithin the range of 30% to 70%. In specific terms, the section period P1maintains the first electrode 15 anode duty ratio for 1 second at 50%which is the 5th stage, and at the next section period P2, the firstelectrode 15 anode duty ratio changes to 55% which is the 6th stage, andthis 55% duty ratio is maintained for 1 second. After that, bysequentially switching from section period P2 through P5, the firstelectrode 15 anode duty ratio changes to 60% of the 7th stage, 65% ofthe 8th stage, and 70% of the 9th stage, respectively. Furthermore, bysequentially switching from section period P5 through P13, the firstelectrode 15 anode duty ratio changes to 70% of the 9th stage, 65% ofthe 8th stage, . . . 30% of the 1st stage 30%, respectively. And bysequentially switching from section period P13 through P17, the firstelectrode 15 anode duty ratio changes to 30% of the 1st stage through50% of the 5th stage. In this way, by increasing and decreasing the dutyratio in stages, it is possible to decrease the heat shock to the tips15 a and 16 a of both electrodes 15 and 16. Here, both the maximum valueDM1 of the anode duty ratio of the first electrode 15 in the anteriorhalf cycle H1 for which the first electrode 15 is biased to the anodeand the maximum value DM2 of the anode duty ratio of the secondelectrode 16 in the posterior half cycle H2 for which the secondelectrode 16 is biased to the anode are equal at 70%.

FIG. 5A and 5B are graphs for describing the waveform of the alternatingcurrent actually supplied to the pair of electrodes 15 and 16. Thehorizontal axis represents time, and the vertical axis represents thecurrent value. Current of a square wave with the fixed current value A0is supplied to both electrodes 15 and 16 at a fixed lighting frequencycorresponding to the alternating current cycle Ta.

As shown in FIG. 5A, with each waveform W5, W6, and W7 during therespective section period P1, P2, and P3 within the anterior half cycleH1 (see FIG. 4), the duty ratio of the alternating current during eachsection period is maintained at the fixed level, and at the time ofswitching of the section periods P1, P2, P3, the duty ratio of eachwaveform W5, W6, and W7 changes to the duty ratio of the 5th, 6th, and7th stages. In specific terms, with the section period P1, when the dutyratio of the waveform W5 when both electrodes 15 and 16 are anodes is50%, at the next section period P2, the duty ratio of the waveform W6when both electrodes 15 and 16 are anodes are respectively 55% and 45%,for example, and with the next section period P3, the duty ratio of thewaveform W7 when both electrodes 15 and 16 are anodes are respectively60% and 40%, for example.

Meanwhile, as shown in FIG. 5B, with each waveform W5, W4, and W3 duringthe respective section period P9, P10, P11 within the posterior halfcycle H2 (see FIG. 4), the duty ratio of the alternating current duringeach section period is maintained at the fixed level, and at the time ofswitching of the section periods P9, P10, P11, the duty ratio of eachwaveform W5, W4, and W3 changes to the duty ratio of the 5th, 4th, and3rd stages. In specific terms, with the section period P9, when the dutyratio of the waveform W5 when both electrodes 15 and 16 are anodes is50%, at the next section period P10, the duty ratio of the waveform W4when both electrodes 15 and 16 are anodes are respectively 45% and 55%,for example, and with the next section period P11, the duty ratio of thewaveform W5 when both electrodes 15 and 16 are anodes are respectively40% and 60%, for example.

As a result, as shown in FIG. 4, within one modulation cycle Tm havingthe anterior half cycle H1 and the posterior half cycle H2, the anodeduty ratio of the first electrode 15 changes cyclically in themodulation range for which the maximum value DMA is 70% and the minimumvalue is 30%, and the anode duty ratio of the second electrode 16changes cyclically in a modulation range for which the maximum value DM2is 70% and the minimum value is 30%.

Note that with the duty ratio modulation such as that shown in FIG. 4and FIG. 5, it is not necessary to maintain at a fixed level thefrequency and current value of the alternating current supplied to bothelectrodes 15 and 16, and it is possible to set different frequenciesand current values for each of the section periods P1, P2, P3, and soon.

Also, with the duty ratio modulation as shown in FIG. 4 and FIG. 5, thesetting values such as the alternating current frequency, the currentvalue, the variable range of the duty ratio, the modulation cycle, thesection period and so on may be changed by increasing or decreasingbased on information relating to the level of consumption of bothelectrodes 15 and 16 and other degradation levels obtained with thedetermination unit 75. For example, when the degradation of bothelectrodes 15 and 16 has progressed, by temporarily increasing ordecreasing the alternating current frequency and the current value, orby increasing the variable range of the duty ratio, it is possible tocredibly melt electrodes for which melting becomes difficult bydegradation over time, and it is possible to maintain shape of theelectrodes to desirable shape over a long time.

FIG. 6A is a graph for describing an exemplary variation of themodulation of the duty ratio shown in FIG. 5. This case is similar toabove in that with each section period P1, P2, P3 within the anteriorhalf cycle H1 of FIG. 4, the duty ratio of the alternating currentduring each section period is maintained at the fixed level, and that atthe time of switching of the section periods P1, P2, P3 the duty ratiochanges to the duty ratio of corresponding stage. Meanwhile, as thewaveform for which the ratio of the time during polarity being positiveor negative to the cycle of the alternating current is greater than orequal to 50%, a superimposed wave is used. In specific terms, at thesection periods P1, P2, P3 through P9 where the anode duty ratio of thefirst electrode 15 is 50% or greater, a superimposed wave for which agradually increasing triangular wave is superimposed on a square wave issupplied to the anode first electrode 15, and the average current valueis maintained at A0, but the peak value of the superimposed wave is A1.Here, the triangular wave rising rate A1/A0, which is defined as theratio of the peak value A1 in relation to the average current value A0,of the superimposed wave is higher than triangular wave rising rate ofthe square wave which is equal to 1. By adjusting the triangular waverising rate, flicker while the electrode works as the cathode may besuppressed, and volume of melting part of the tip while the electrodeworks as the anode may increase. As a consequence, the discharge may bestabilized and the shape of the electrode is maintained in the desiredshape.

FIG. 6B corresponds to FIG. 6A, but shows each section period P9, P10,P11 within the posterior half cycle H2 of FIG. 4. This case is alsosimilar as the case of FIG. 5B in that the duty ratio of the alternatingcurrent during each section period is maintained at a fixed level, andthat at the time of switching of the section periods P9, P10, P11 theduty ratio changes to the duty ratio of corresponding stage. Meanwhile,as the waveform for which the ratio of the time during polarity beingpositive or negative to the cycle of the alternating current is greaterthan or equal to 50% or greater, a superimposed wave is used. Inspecific terms, at the section periods P9, P10, P11 through P16 wherethe anode duty ratio of the second electrode 16 is 50% or greater, asuperimposed wave for which a gradually increasing triangular wave issuperimposed on a square wave is supplied in the half cycle where thesecond electrode 16 works as the anode, and the average current value ismaintained at A0, but the peak value of the superimposed wave is A1.Here, the triangular wave rising rate A1/A0 is higher than thetriangular wave rising rate of the square wave which is equal to 1. Byadjusting the triangular wave rising rate, flicker while the electrodeworks as the cathode may be suppressed, and volume of melting part ofthe tip part while the electrode works as the anode may increase. As aconsequence, the discharge may be stabilized and the shape of theelectrode is maintained in the desired shape.

Note that it is possible to combine the operations of FIG. 6A and FIG.6B, but it is also possible to use only the operation of FIG. 6A orFIG.6B. For example, it is possible to use the square wave for theanterior half cycle H1 where the first electrode 15 anode period isrelatively long, and to use the superimposed wave shown in FIG. 6B onlyfor the posterior half cycle H2 where the second electrode 16 anodeperiod is relatively long.

In the above exemplary variation, as the waveform for which the ratio ofthe time during polarity being positive or negative to the cycle of thealternating current is greater than or equal to 50%, a superimposedwave, which a gradually increasing triangular wave is superimposed onthe basic square wave, is used to vary the current. It is also possibleto vary the current based on the information relating to the degradationstages of both electrodes 15 and 16 and the like. In addition, it isalso possible to vary the current by changing waveforms so that thecurrent value of the waveforms is maximum at the end of the polarityperiod where the time ratio of the polarity to one cycle of thealternating current is greater than or equal to 50%, and bysuperimposing various types of waveforms such as triangular waves thatincrease in the posterior half of the half cycle, square waves, and sinewaves on the basic square wave.

Also, for modulation of the duty ratio like that shown in FIG. 6A and6B, it is possible to change the setting values such as the duty ratiovariation range, modulation cycle length, section period length,lighting frequency, current value and the like based on the consumptionlevel and other information relating to the degradation stage of bothelectrodes 15 and 16 obtained by the determination unit 75.

FIG. 7 is a flow chart for describing the operation of the light sourcedriver 70. The control unit 70 b reads suitable initial operation datanecessary for lighting start of the discharge lamp 1 (step S11) from theoperation control table stored in the data storage unit 76.

Next, the control unit 70 b controls the lighting unit 70 a based on theinitial power feed conditions read at step S11, and controls the initialoperation including the run up operation from the startup of thedischarge lamp 1 (step S12).

Next, the control unit 70 b reads suitable steady operation datanecessary for maintaining light emission of the discharge lamp 1 fromthe operation control table stored in the data storage unit 76 (stepS13). In specific terms, it reads setting values of the steady operationtime such as the current value, the lighting frequency, the duty ratiovariable range, the modulation cycle length, the section period length,the triangular wave rising rate and the like. At this time, a modulationpattern including the lighting waveform such as the current value andthe lighting frequency, the duty ratio variation range, the modulationcycle length, the section period length or the like is selected based onthe consumption level and other information relating to the degradationstage of both electrodes 15 and 16 obtained by the determination unit75.

Next, the control unit 70 b controls the rated operation state of thelighting unit 70 a, specifically, the steady operation of the dischargelamp 1 (step S14) based on the steady power feed conditions read at stepS13.

Here, during steady operation, the determination unit 75 determineswhether or not the interrupt request signal requesting the end of thelighting operation of the light source unit 10 has been input (stepS15). When this kind of interrupt request signal has been input,information indicating the recent state of the discharge lamp 1 such asthe recent cumulative lighting time, the recent voltage supplied to thedischarge lamp 1 and the like is recorded in the data storage unit 76,and the process moves to the light turn-off operation.

As is clear from the description above, with the light source device 100of this embodiment, with the steady operation for which the dischargelamp 1 has rated operation done by the lighting unit 70 a which operatesunder the control of the control unit 70 b, the duty ratio of thealternating current supplied between the first and second electrodes 15and 16 is changed by a specified modulation pattern having sectionperiods P1, P2, P3, . . . for which the same value is maintained for 1second or more. By doing this, even while changing the alternatingcurrent duty ratio, it is possible to temporarily fix the duty ratio fora fixed period, so the alternating current supplied between bothelectrodes 15 and 16 is not steady in relation to the polarity, and itis possible to greatly fluctuate the heat state of both electrodes andtheir periphery over a relatively long time scale. Thus, it is possibleto avoid the formation of a steady convection flow in the discharge lamp1, and it is possible to suppress degradation of the electrode shape ofboth electrodes 15 and 16 and biased precipitation of the electrodematerials. By doing this, it is possible to maintain the illuminationlevel of the illumination light from the discharge lamp 1 and tolengthen the product life of the discharge lamp 1 or the light sourcedevice 100.

FIG. 8 is a schematic drawing for describing the configuration of aprojector incorporating the light source device 100 of FIG. 1. Theprojector 200 has the light source device 100, the illumination opticalsystem 20, a color separation optical system 30, a light modulating unit40, a cross dichroic prism 50, and a projection lens 60. Here, the lightmodulating unit 40 includes three liquid crystal light valves 40 a, 40b, and 40 c having the same configuration.

With the aforementioned projector 200, the light source device 100 isequipped with the light source unit 10 and the light source driver 70shown in FIG. 1. The light source device generates illumination lightfor illuminating the light modulating unit 40, i.e. the liquid crystallight valves 40 a, 40 b, and 40 c via the illumination optical system 20etc.

The illumination optical system 20 has a parallelizing lens 22 whichparallelizes the light from the source light into the light fluxdirection, a first and second fly eye lens 23 a and 23 b constitutingthe integrator optical system for dividing and superimposing light, apolarization conversion component 24 which aligns the polarizationdirection of the light, and a superimposing lens 25 which superimposesthe light via both fly eye lenses 23 a and 23 b. With the illuminationoptical system 20, the image forming area of the liquid crystal lightvalves 40 a, 40 b, and 40 c are illuminated by illumination light withan uniform illumination intensity. With the illumination optical system20, the parallelization lens 22 converts the light flux direction of theillumination light emitted from the light source unit 10 to be roughlyparallel. The first and second fly eye lenses 23 a and 23 b respectivelyconsist of a plurality of element lenses arranged in matrix form, andthe light via the parallelization lens 22 is divided and convergedindividually by the element lens constituting the first fly eye lens 23a, and the divided light flux from the first fly eye lens 23 a isemitted at a suitable divergence angle by the element lens constitutingthe second fly eye lens 23 b. The polarization conversion component 24is formed by an array which has a PBS, a mirror, and a phase differenceplate and the like as a set of elements. The polarization conversioncomponent 24 aligns the polarization direction of the light flux of eachpart divided by the first fly eye lens 23 a to linear polarized light ofone direction. The superimposing lens 25 suitably converges entirety ofthe illumination light via the polarization conversion component 24.This allows the illuminated area of the liquid crystal light valves 40a, 40 b, and 40 c which are the light modulation devices of each colorof the latter stage to be illuminated by superimposed light.

The color separation optical system 30 has first and second dichroicmirrors 31 a and 31 b, reflective mirrors 32 a, 32 b, and 32 c, andthree field lenses 33 a, 33 b, and 33 c. The illumination light emittedfrom the illumination light optical system 20 is bended by the mirror 26and divided into the three color lights of red (R), green (G), and blue(B). Each of the color light is led to the latter stage liquid crystallight valves 40 a, 40 b, and 40 c. To describe this in more detail,first, of the three colors RGB, the first dichroic mirror 31 a transmitsthe R light and reflects the G light and B light. Also, of the twocolors GB, the second dichroic mirror 31 b reflects G light andtransmits B light. Next, with this color separation optical system 30,the R light transmitted through the first dichroic mirror 31 a incidentsinto the field lens 33 a for adjusting the incident angle via thereflective mirror 32 a. Also, the G light reflected by the firstdichroic mirror 31 a and the second dichroic mirror 31 b incident intothe field lens 33 b for adjusting the incident angle. Furthermore, the Blight transmitted through the second dichroic mirror 31 b incidents intothe field lens 33 c for adjusting the incident angle via relay lensesLL1 and LL2 and the reflective mirrors 32 b and 32 c.

The liquid crystal light valves 40 a, 40 b, and 40 c that constitute thelight modulating unit 40 are non-light emitting type light modulationdevice for modulating spatial intensity distribution of the incidentillumination light. The liquid crystal light valves 40 a, 40 b, and 40 care equipped with three liquid crystal panels 41 a, 41 b, and 41 crespectively illuminated corresponding to each color light emitted fromthe color separation optical system 30, three first polarization filters42 a, 42 b, and 42 c respectively arranged at the incidence side of eachliquid crystal panel 41 a, 41 b, and 41 c, and three second polarizationfilters 43 a, 43 b, and 43 c arranged respectively at the emitting sideof each liquid crystal panel 41 a, 41 b, and 41 c. The R lighttransmitted through the first dichroic mirror 31 a incidents into theliquid crystal light valve 40 a via the field lens 33 a etc., andilluminates the liquid crystal panel 41 a of the liquid crystal lightvalve 40 a. The G light reflected by both the first and second dichroicmirrors 31 a and 31 b incidents into the liquid crystal light valve 40 bvia the field lens 33 b etc., and illuminates the liquid crystal panel41 b of the liquid crystal light valve 40 b. The B light reflected bythe first dichroic mirror 31 a and transmitted through the seconddichroic mirror 31 b incidents into the liquid crystal light valve 40 cvia the field lens 33 c etc., and illuminates the liquid crystal panel41 c of the liquid crystal light valve 40 c. Each liquid crystal panel41 a to 41 c modulates the spatial distribution of the polarizationdirection of the illumination light, and the polarized light status ofthe three color lights incident respectively on each liquid crystalpanel 41 a to 41 c is adjusted pixel-by-pixel according to a drivesignal or image signal input as an electrical signal to each of theliquid crystal panels 41 a to 41 c. At this time, the polarizationdirection of the illumination light which incident into each liquidcrystal panel 41 a to 41 c is adjusted by the first polarization filters42 a to 42 c, and modulated light of a specified polarization directionis extracted from the modulated light emitted from each liquid crystalpanel 41 a to 41 c by the second polarization filters 43 a to 43 c. Bythe above, each liquid crystal light valve 40 a, 40 b, and 40 c formseach color image light corresponding respectively.

The cross dichroic prism 50 synthesizes each color image light from eachliquid crystal light valve 40 a, 40 b, and 40 c. To explain this in moredetail, the cross dichroic prism 50 is made in a plan view roughlysquare shape with four right angle prisms adhered together, and at theinterface where the right angle prisms are adhered together, a pair ofdielectric multi layer films 51 a and 51 b that intersect in an X shapeare formed. One first dielectric multi layer film 51 a reflects the Rlight, and the other second dielectric multi layer film 51 b reflectsthe B light. With the cross dichroic prism 50, the R light from theliquid crystal light valve 40 a is reflected by the dielectric multilayer film 51 a and emitted to the progression direction right side, theG light from the liquid crystal light valve 40 b is advanced straightahead and emitted via the dielectric multi layer films 51 a and 51 b,and the B light from the liquid crystal light valve 40 c is reflected bythe dielectric multi layer film 51 b and emitted to the progressiondirection left side. Working in this way, the R light, G light, and Blight are synthesized by the cross dichroic prism 50, and a synthesizedlight which is an image light is formed with a color image.

The projection lens 60 is a projection optical system that enlarges thesynthesized light formed via the cross dichroic prism 50 as the imagelight at a desired enlargement ratio and projects a color image on ascreen (not illustrated).

With the projector 200 described above, it is possible to suppressdegradation of the electrode shape and biased precipitation of electrodematerials of the pair of electrodes 15 and 16 that constitute the lightsource device 100, and it is possible to maintain the projectionbrightness of the projector 200 over a long period.

Second Embodiment

Following, the light source device as the second embodiment will bedescribed. Note that the light source device of the second embodiment isa variation of the light source device 100 of the first embodiment, andparts that are not specifically described are the same as those of thelight source device 100 of the first embodiment.

FIG. 9 is a graph for describing the modulation of the duty ratio of thealternating current supplied to the pair of electrodes 15 and 16. Thehorizontal axis represents time, and the vertical axis represents dutyratio. With this modulation pattern of the alternating current suppliedto both electrodes 15 and 16, the section period DP1 for which the firstelectrode 15 anode duty ratio is greater than or equal to 50% where thefirst electrode 15 anode period is relatively long, and the sectionperiod DP2 for which the first electrode 15 anode duty ratio is less orequal to 50% where the second electrode 16 anode period is relativelylong are alternately repeated. As is also clear from the graph, thefirst electrode 15 anode duty ratio and the second electrode 16 anodeduty ratio both change in a modulation range from the minimum value of30% to the maximum value of 70%.

As shown in FIG. 9, by alternately repeating the section period DP1 forwhich the first electrode 15 anode duty ratio is greater than or equalto 50%, and the section period DP2 for which the first electrode 15anode duty ratio is less than 50%, it is possible to enlarge changingamount in the duty ratio between two consecutive section periods(hereafter also simply referred to as “duty ratio changing amount”).

Note that with the modulation pattern shown in FIG. 9, the duty ratiochanging amount gradually increases with the anterior half of themodulation period Tm, and gradually decreases in the posterior half ofthe modulation period Tm. As the modulation pattern, it is also possibleto use various patterns as long as the pattern alternately repeats asection period for which one electrode anode duty ratio is greater thanthe reference duty ratio, which is the median value of the modulationrange of the anode duty ratio of the concerned electrode (with theexample in FIG. 9, 50%), and a section period for which the duty ratiois less than the reference duty ratio. For example, it is possible touse a modulation pattern that alternately repeats a section period forwhich the first electrode 15 anode duty ratio is 60%, and a sectionperiod for which the first electrode 15 anode duty ratio is 40%.However, since it is possible to more effectively suppress convectionflow AF being located inside the discharge lamp 1, it is desirable tochange the duty ratio changing amount within the modulation cycle Tm asshown in FIG. 9. Note that with the example in FIG. 9, the referenceduty ratio is the median value of the modulation range, but thereference duty ratio may also to be a predetermined value based on themedian value of the modulation range. The reference duty ratio may beset by shifting from the median value in accordance with the dischargelamp 1 characteristics, the length of each section period, or thewaveform of the alternating current or the like. For example, when theperiod for which the anode duty ratio is higher than the median value islonger than the period that is lower than the median value for theentire modulation cycle, it is possible to set the reference duty ratiohigher than the median value.

FIG. 10A through FIG. 12B are explanatory drawings showing the effect ofthe duty ratio changing amount on the tips 15 a and 16 a of theelectrodes 15 and 16. FIG. 10A, FIG. 11A, and FIG. 12A show modulationpatterns of alternating current when the duty ratio changing amount ADis set to 5%, 10%, and 20% respectively. The horizontal axis of each ofthese graphs represents time, and the vertical axis represents thealternating current duty ratio. FIG. 10B, FIG. 11B, and FIG. 12B showchanging of the shape of the tips 15 a and 16 a and the large-diameterportions 15 b and 16 b of the electrodes 15 and 16 when the modulationpatterns shown in FIG. 10A, FIG. 11A, and FIG. 12A were used,respectively. In FIG. 10B, FIG. 11B, and FIG. 12B, the solid lineindicates the electrode shape after the discharge lamp 1 was operatedfor 65 hours, and the dot-dash line indicates the electrode shape in astate when the discharge lamp 1 was unused.

As shown in FIG. 10B, when the modulation pattern shown in FIG. 10A wasused, specifically, when the duty ratio changing amount AD was 5%, thesize of the tips 15 a and 16 a of the electrodes surrounded by thedotted line was almost the same as the unused state (dot-dash line). Asshown in FIG. 11B, when the duty ratio changing amount AD was 10% (FIG.11A), the size of the tips 15 a and 16 a of the electrodes surrounded bythe dotted line was larger than when the duty ratio changing amount ADwas 5%. Furthermore, when the duty ratio changing amount AD was 20%(FIG. 12A), the size of the tips 15 a and 16 a of the electrodessurrounded by the dotted line was even larger than when the duty ratiochanging amount AD was 10%. In this way, the size of the tips 15 a and16 a of the first and the second electrodes after the discharge lamp 1was operated became larger in accordance with the duty ratio changingamount AD becoming larger.

As shown in FIG. 1OA through FIG. 12B, as the duty ratio changing amountAD became larger, the tips 15 a and 16 a of the electrodes 15 and 16grew even larger with operation. From this result, by making the dutyratio changing amount AD greater than a specified value (e.g. 7.5%), thetips 15 a and 16 a grow with operation, and we found that it is possibleto suppress the flattening of the tips 15 a and 16 a.

With the second embodiment, by alternately repeating the section periodDP1 for which the first electrode 15 anode duty ratio is greater than orequal to 50% where the first electrode 15 anode period is relativelylong, and the section period DP2 for which the first electrode 15 anodeduty ratio is less than 50% where the second electrode 16 anode periodis relatively long, the duty ratio changing amount is made larger.Because of this, with the second embodiment, it is possible to grow thetips 15 a and 16 a with operation, and it is possible to suppressdegradation of the electrode shape such as flattening of the tips 15 aand 16 a or the like.

Third Embodiment

Following, the light source device of the third embodiment will bedescribed. Note that the light source device of the third embodiment isa variation of the light source device 100 of the first embodiment, andparts not specifically described are the same as the light source device100 of the first embodiment.

FIG. 13 is a graph for describing the modulation of the duty ratio ofthe alternating current supplied to the pair of electrodes 15 and 16.The horizontal axis represents time, and the vertical axis representsthe duty ratio. In this case, the modulation pattern of the alternatingcurrent supplied to both electrodes 15 and 16 consists of the anteriorhalf period H1 for which the first electrode 15 anode duty ratio isgreater than or equal to 50% where the anode period of the firstelectrode 15 is relatively long, and the posterior half period H2 forwhich the first electrode 15 anode duty ratio is less or equal to 50%where the anode period of the second electrode 16 is relatively long.The modulation pattern of the third embodiment differs from themodulation pattern shown in FIG. 4 that the duty ratio uniformlyincreases and decreases with a fixed difference at each stage in thatthe changing amount of the duty ratio changes in time.

With the third embodiment as well, the duty ratio changing amountbetween the section periods P1 and P2 becomes larger. Because of this,the same as with the second embodiment, it is possible to grow the tips15 a and 16 a with operation, and it is possible to suppress degradationof the electrode shape such as flattening of the tips 15 a and 16 a.

Variation of Modulation Pattern The modulation patterns shown in FIG. 4,FIG. 9, and FIG. 13 are only examples, and by changing the alternatingcurrent supplied to the pair of electrodes 15 and 16 using variousmodulation patterns, it is possible to prevent excessive localization ofconvection flow AF inside the discharge lamp 1. Also, as shown in FIG. 9and FIG. 13, by making the duty ratio changing amount greater than aspecified value, it is possible to suppress degradation of the electrodeshape. For example, it is also possible to change the alternatingcurrent using the following modulation patterns.

First Variation

FIG. 14 is an explanatory drawing showing a first variation of themodulation pattern. With the modulation pattern of the first variation,in the anterior half of the modulation cycle Tm, the period for whichthe first electrode 15 anode duty ratio is less than (low duty ratioperiod) the reference duty ratio (50%) is shortened, and in theposterior half of the modulation cycle Tm, the period for which theanode duty ratio exceeds the reference duty ratio (high duty ratioperiod) is shortened. The other points are the same as the modulationpattern of the second embodiment shown in FIG. 9.

In a state for which the anode duty ratio of one electrode is high, thetemperature of that electrode rises. In this way, in a state when thetemperature rises, when the electrode operates as a negative electrode,there is a large amount of emission (sputter) of electrode material intothe discharge space 12 due to collision of positive ions (e.g. Ar+ orHg+) generated by discharge, and it is easy for blackening of the innerwall of the discharge space 12 to occur. In light of this, with thefirst variation, at the anterior half of the modulation cycle Tm wherethe temperature of the first electrode 15 rises, the low duty ratioperiod is shortened thereby suppressing the occurrence of sputter, andat the posterior half of the modulation cycle Tm where the temperatureof the second electrode 16 rises, the high duty ratio period isshortened thereby suppressing the occurrence of sputter.

Meanwhile, even with the first variation, by alternately repeating thesection period for which the anode duty ratio of the first electrode 15is greater than or equal to 50% where the first electrode 15 anodeperiod is relatively long, and the section period for which the anodeduty ratio of the first electrode 15 is less or equal to 50% where thesecond electrode 16 anode period is relatively long, the duty ratiochanging amount becomes greater. Because of this, it is possible to growthe tips 15 a and 16 a with operation, and it is possible to suppressdegradation of the electrode shape such as flattening of the tips 15 aand 16 a and the like.

Second Variation

FIG. 15 is an explanatory drawing showing a second variation of themodulation pattern. With the modulation pattern of the second variation,the changing amount of the duty ratio from the high duty ratio period ofthe first electrode 15 to the low duty ratio period following theconcerned high duty ratio period is a fixed value (with the example inFIG. 15, 30%), and overall, the duty ratio changes gradually with themodulation cycle Tm (8 seconds). In this way, with the second variation,the duty ratio changing amount is sufficiently large, so the tips 15 aand 16 a grow with operation, and degradation of the electrode shapesuch as flattening of the tips 15 a and 16 a is suppressed. Also, it ispossible to gradually fluctuate the heat state of both electrodes 15 and16 and their vicinity with a long span so as to affect the convectionflow AF. Thus, formation of a steady convection flow AF inside the tubebody 11 of the discharge lamp 1 may be avoided.

Others:

The present invention is not limited to the examples and embodimentsdescribed above and may be reduced to practice in various forms withoutdeparting the scope thereof including, for example, the followingmodifications.

As the lamp of the light source unit 10 in the embodiments describedabove, various lamp such as a high pressure mercury lamp, a metal halidelamp or the like may be used. It is also possible to use a light sourceof a type that does not have the sub-mirror 3 as the light source unit10.

In the projector 200 of the embodiments described above, a pair of flyeye lenses 23 a and 23 b for dividing the light from the light sourcedevice 100 into a plurality of partial light flux is used, but thisinvention can also be used for a projector that does not use this kindof fly eye lens, i.e. lens array. Furthermore, it is also possible toreplace the fly eye lenses 23 a and 23 b with a rod integrator.

In the aforementioned projector 200, a polarization conversion component24 that polarizes the light from the light source device 100 to aspecific direction is used, but this invention may also be applied to aprojector that does not use this kind of polarization conversioncomponent 24.

In the embodiments described above, an example is described where thepresent invention is applied to a transmission type projector, but it isalso possible to apply the present invention to a reflective typeprojector. Here, the “transmission type” means that the projector isequipped with a liquid crystal light valve including a liquid crystalpanel which transmits light. The “reflective type” means that theprojector is equipped with a liquid crystal light valve which reflectslight. Note that the light modulation device is not limited to a liquidcrystal panel. For example, a light modulation device with a micromirror may also be used.

As the projector, there are a front side projector that projects imageprojection from the direction observing the projection surface, and aback side projector that projects image from the opposite side from thedirection observing the projection surface. The configuration of theprojector shown in FIG. 8 may be applied to either of these projectors.

In the embodiments described above, only an example of a projector 200using three liquid crystal panels 41 a through 41 c is presented. Thepresent invention may also be applied to a projector using only oneliquid crystal panel, a projector using two liquid crystal panels, or aprojector using four or more liquid crystal panels.

In the embodiments described above, each color light is modulated usingthe color separation optical system 30 and the liquid crystal lightvalves 40 a, 40 b, and 40 c. It is also possible to perform color lightmodulation and synthesis instead of this. For example, modulation andsynthesis of the color lights may be performed by combining a colorwheel illuminated by the light source device 100 and the illuminationoptical system 20, and a device equipped with micro mirror pixels forwhich transmitted light of the color wheel is radiated.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A light source device comprising: a discharge lamp that emits lightby discharge between a first electrode and a second electrode; and adriver that supplies alternating current to the first and the secondelectrodes so as to maintain the discharge, and changes duty ratio ofthe alternating current in accordance with predetermined pattern, thepredetermined pattern including a plurality of section periods for whichthe duty ratio maintains mutually different values for a predeterminedperiod.
 2. The light source device according to claim 1 wherein lengthof the predetermined period is between 0.1 second and 1 minute.
 3. Thelight source device according to claim 1 wherein one cycle of thealternating current includes a positive polarity period and a negativepolarity period for which current value is positive and negativerespectively, and the driver varies the current value during a longerpolarity period among the positive and the negative polarity periods,time ratio of the longer polarity period to the one cycle of thealternate current being at least 50%.
 4. The light source deviceaccording to claim 3 wherein the driver varies the current value duringthe longer polarity period so that the current value is at a maximum atan end of the longer polarity period.
 5. The light source deviceaccording to claim 1, wherein the driver increases current value whileat least one of the first and the second electrodes works as anodetogether with time.
 6. The light source device according to claim 1wherein the difference of the duty ratio between a first section periodamong the plurality of section periods and a second section period amongthe plurality of section periods following the first section period isgreater than a predetermined amount.
 7. The light source deviceaccording to claim 6 wherein the duty ratio in the first section periodand the duty ratio in the second section period are changed so as toextend across a reference duty ratio determined in accordance medianvalue of changing range of the duty ratio.
 8. The light source deviceaccording to claim 7 wherein length of the first section period andlength of the second section period are mutually different.
 9. The lightsource device according to claim 8 wherein the predetermined pattern isa pattern that cyclically changes, in a specified period within onecycle of the pattern, a section period for which the duty ratio ishigher than the reference duty ratio is longer than a section period forwhich the duty ratio is lower than the reference duty ratio, and in aremaining period within the one cycle of the pattern, a section periodfor which the duty ratio is higher than the reference duty ratio isshorter than a section period for which the duty ratio is lower than thereference duty ratio.
 10. A projector comprising: a discharge lamphaving a first electrode and a second electrode that emits light bydischarge between the first and the second electrodes; a driver thatsupplies alternating current to the first and the second electrodes soas to maintain the discharge, and changes duty ratio of the alternatingcurrent in accordance with predetermined pattern, the predeterminedpattern including a plurality of section periods for which the dutyratio maintains mutually different values for a predetermined period; alight modulating device illuminated by illumination light from thedischarge lamp; and a projection optical system that projects imageformed by the light modulating device.
 11. A drive method of a dischargelamp having a first electrode and a second electrode comprising:supplying alternating current to the first and the second electrodes soas to maintain discharge between the first and the second electrodes foremission of light; and changing duty ratio of the alternating current inaccordance with predetermined pattern, the predetermined patternincluding a plurality of section periods for which the duty ratiomaintains mutually different values for a predetermined period.